Cystic fibrosis (CF) is an inherited multisystem disease characterized by progressive deterioration in lung function and pancreatic insufficiency attributed to dysfunction of a single gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR). Although CF is considered a monogenic disorder, phenotype expression is considerably diverse even in patients with the same CFTR mutation. Patients with the most common mutation, delta 508 a deletion of a phenylalanine at position 508 of CFTR, often have markedly different clinical courses; some, have less aggressive lung disease and survive into their 50s, while others have a precipitous decline in lung function and die of respiratory failure in their early 20s. What accounts for this phenotypic heterogeneity is unclear. The goal of this proposal is to identify non-CFTR candidate genes that may impact the severity of CF lung disease and account for phenotypic heterogeneity. Any modifier gene identified will have important implications for defining the pathophysiology of CF lung disease, stratifying patients, and identifying new targets for therapy. To initially test our hypothesis, we propose to evaluate candidate genes that are immune-related and non-CFTR genes. Our strategy will use a family based association analysis to test for association of candidate genes to severity of pulmonary disease. We propose to combine new genotyping technology, well-powered samples, and a haplotype- based approach to comprehensively and definitively determine variation in the most promising candidate genes modifying CF lung disease. A unique contribution of this research will the examination of genetic modifiers in CF in parent child-trios to evaluate polymorphisms in genes that may impact phenotype and hence CF lung disease. The overall hypothesis to be tested is that polymorphisms in genes associated with a well-defined phenotype represent modifying factors that account for the variability in expression of CF lung disease as measured by lung function, forced expiratory volume in one second (FEV1), in patients with the delta 508 genotype. To test this global hypothesis, we have established three specific aims: 1) Establish and characterize a CF database and define key features of the quantitative and qualitative components of our CF phenotype; relate phenotype to variation in disease severity as defined by levels of FEV1 adjusted for age and gender in CF patients homozygous for delta F508 CFTR allele. 2) Genotype single nucleotide polymorphic markers (SNPs) from five promising genes (selected based on data from Aim 1, the PGA, microarray analysis, and the literature) found to be associated with lung function in a sample of 100 individuals homozygous for delta 508 and identify common haplotypes and htSNPs that tag these haplotypes for these genes. 3) Genotype these htSNPs identified in AIM 2 in a sample of homozygous delta F508 individuals and their parents, and perform family based association analysis for CF phenotype and pulmonary function.